Extreme heat on the rise

Peak high temperatures on land continue to climb despite Earth’s stalled averages

SWELTERING SWING In the last 15 years, the hottest temperatures on land got hotter (red, with trend line), while multiple datasets of global weather indicate that mean temperatures mostly stayed steady (blue and black). The blue data and trend line are from the European Center for Medium-Range Weather Forecasts and the black data and trend line are from the Met Office Hadley Centre.

A surge of heat spikes has struck land in the last 15 years or so, despite an overall plateau of global surface temperatures in the same period.

Researchers are still deciphering the reasons for Earth’s warming hiatus (SN Online: 2/9/14). But the boost in high terrestrial temperatures attests to continuing climate change, scientists argue in the March Nature Climate Change.

In an analysis led by climate scientist Sonia Seneviratne of ETH Zürich, researchers combed through global weather records from 1979 to 2010. The team assessed the area of land experiencing peak highs, defined as days above the 90th percentile of a region’s typical temperature range over the three decades.

Despite a plateau in mean global temperatures, the authors report a clear rise in the area of land experiencing more than 30 days of harsh heat a year. The pattern is even clearer when the researchers look at the number of regions experiencing more than 50 days of severe heat.

The results, the authors argue, “show that it would be erroneous to interpret the recent slowdown of the global annual mean temperature increase as a general slowdown of climate change.”

Atmospheric scientist Dick Dee of the European Centre for Medium-Range Weather Forecasts in Reading, England, agrees. The mean temperature or any other single variable, he says, is “not enough to talk about climate change.” Instead, he says, “You have to look at many different things.”

The finding that terrestrial heat extremes are rising is interesting but not surprising, says climate scientist Michael Mann of Pennsylvania State University in University Park. “This is certainly true in the U.S., where 2012 was the warmest year on record, and where we've suffered devastating heat waves over the past several summers,” he says.

Together with other weather and climate observations, Mann adds, the new data are the latest reminder that climate change continues unabated.